Try to be strictly correct when using fall back vector (vec_fb) in function like memcpy/memcmp

The vec_fb is essentially a struct with one member m_arr.

using memcpy/memcmp from array to struct etc is a bit iffy due to padding .

I think is fine if there is only one data member that is the array (at least the tests seemed to work) but better be safe than sorry,

so use the array with the payload directly.

ping @ssnyder